In the course of a systematic study of the structural and conformational phase-transition sequences present in crystals with the general formula ${\left({\mathrm{C}}_n{\mathrm{H}}_{2n+1}{\mathrm{NH}}_3\right)}_2\mathrm{Zn}{\mathrm{Cl}}_4$, a pure proper gyrotropic phase transition has been discovered at 249 K in the bis-(pentylammonium) tetrachlorozincate (II) crystals. To the best of our knowledge this is the first report of a phase transition of such type. In order to characterize it, the crystal structures below and above 249 K have been determined by x-ray diffraction analysis and thermal and optical measurements have been made. The transition is equitranslational and occurs between space groups Pnma ($D_{2h}^{16}$) and $P{2}_1{2}_1{2}_1$ ($D_2^4$). No ferroelectric or ferroelastic behavior is then associated with the distorted phase, the spontaneous onset of optical activity and piezoelectricity being its main characteristics. This type of transition where the gyrotropic tensor is the lowest-rank tensor characterizing the ferroic phase (pure gyrotropic) had been theoretically predicted but no experimental case had been studied until now. Thermal expansion, birefringence, and specific-heat measurements imply a first-order character for the transition. The gyrotropic domains corresponding to the two opposite possible values of the rotatory power could be observed and a magnitude proportional to the rotatory power (order parameter) was measured as a function of the temperature. The experimental data give evidence of a Landau-type behavior for the gyrotropic coefficients in the low-temperature phase. A comparison with compounds belonging to the same family with shorter organic chains suggests that similar microscopic mechanisms in all these compounds lead to macroscopic order parameters of different symmetry and consequently to a phase transition of very different characteristics.

• Received 24 May 1983

DOI:https://doi.org/10.1103/PhysRevB.29.2655